1. Field of the Invention
The invention relates to a rotational angle detection device and an electric power steering system.
2. Description of the Related Art
Some vehicle power steering systems execute a power assist control based on the steering angle formed by a steering wheel. Examples of power assist control executed based on the steering angle (steering speed) include a steering wheel return control for returning a steering wheel to the neutral position and a damping compensation control for restricting an abrupt change in the steering angle. The steering angle is usually detected by using, as a steering sensor, a rotational angle sensor that outputs ABZ-phase pulse signals as described in Japanese Patent Application Publication No. 6-127417 (JP-A-6-127417).
However, such a pulse-type rotational angle sensor has a problem that it is difficult to achieve both an increase in the detection accuracy and downsizing. Therefore, for example, Japanese Patent Application Publication No. 2007-256250 (JP-A-2007-256250) suggests a rotational angle sensor that includes a magnetic detection element and is able to output a sine signal and a cosine signal of which the output levels change in accordance with the rotational angle of a detection target. If such a magnetic rotational angle sensor is used as a steering sensor, it is possible to detect the steering angle with a high degree of accuracy while downsizing the steering sensor.
The rotational angle that is detected based on signals output from a rotational sensor is basically a relative angle (electrical angle) of a rotary shaft, which is a detection target, within a range of 360° in mechanical angle, whereas the steering angle formed by the rotation of a steering wheel is an absolute angle that may fall outside the range of 360° in mechanical angle. Therefore, in many cases, the steering angle is detected by counting changes in the relative angle that is detected based on the signals output from a steering sensor.
However, in order to detect the rotational angle based on a sine signal and a cosine signal as described above, it is necessary to execute A/D conversion or a complicated calculation process which is not required in the conventional rotational angle detection based on pulse signals. Therefore, if the above-described counting of changes in the relative angle is continued even after an ignition is turned off in order to ensure the continuity of the steering angle detection within a period from when the ignition is turned off until when the ignition is turned on again, the power consumption increases, resulting in acceleration of exhaustion of an in-vehicle power supply.
To address this problem, each of JP-A-2007-256250 and Published Japanese Translation of PCT application No. 2007-533975 suggests a rotational angle detection device in which the axial distance between a magnetic rotor and a magnetic detection element that constitute a rotational angle sensor is changed based on the absolute angle. Thus, the vector length of the density of magnetic flux that passes through the magnetic detection element is continuously changed based on the absolute angle. As a result, it is possible to detect the absolute angle without counting changes in the rotational angle.
However, providing such a mechanical configuration for changing the above-described axial distance complicates the structure, resulting in an increase in the production cost. Further, the presence of such a movable portion where the axial distance is changed may become a factor of reduction in the reliability. On this point, there is still room for improvement.